1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a method and an apparatus for transmitting and receiving control information.
2. Description of the Related Art
In the field of mobile communication technology, active research for an Orthogonal Frequency Division Multiple Access (OFDMA) scheme or a Single Carrier-Frequency Division Multiple Access (SC-FDMA) scheme similar to the OFDMA scheme is being conducted as a scheme useful for high-speed data transmission through a wireless channel. The 3rd Generation Partnership Project (3GPP), which is an organization for asynchronous cellular mobile communication standardization, is currently researching a Long Term Evolution (LTE) system, which is a next generation mobile communication system, as a basis for the multiple access schemes.
In the LTE system, transport formats of uplink control information are divided according to an existence or absence of data transmission. The uplink control information includes ACKnowledgement (ACK)/Negative ACKnowledgement (NACK) information as a response to downlink data transmission, Channel Quality Indication (CQI) information for feeding back a downlink channel state, and Multiple Input Multiple Output (MIMO) information necessary to operate multiple transmission/reception antennas.
Either when data and control information are simultaneously transmitted or when only data is transmitted in an uplink, the data and the control information are multiplexed before transmission. In contrast, when only control information is transmitted without data, a particular allocated frequency band is used to transmit the control information.
FIG. 1 illustrates a structure of control information when only the control information is transmitted in an uplink in a 3GPP LTE system. In FIG. 1, the horizontal axis indicates the time domain and the vertical axis indicates the frequency domain. The time domain has a range of one sub-frame 102 and the frequency domain has a range of a transmission bandwidth 114.
Referring to FIG. 1, the sub-frame 102, which is a basic transmission unit of uplink, has a length of 1 ms, and each sub-frame includes two slots 104 and 106, each of which has a length of 0.5 ms. Each of the slots 104 and 106 includes a plurality of Long Blocks (LBs) 108, each of which is also called a Long SC-FDMA Symbol. Each of the slots shown in FIG. 1 includes seven LBs 108.
In the frequency domain, the smallest transmission unit is a sub-carrier, and a basic unit for resource allocation is a Resource Unit (RU) 110 or 112. The RU 110 or 112 includes a plurality of sub-carriers and a plurality of LBs. In the structure shown in FIG. 1, one RU includes 12 sub-carriers and 14 LBs. In the structure, one RU may include not only continuous sub-carriers but also discontinuous sub-carriers having a regular interval between them, so as to obtain frequency diversity.
Within one sub-frame 102, control information is transmitted at the 1st, 2nd, 3rd, 5th, 6th, 7th, 8th, 9th, 10th, 12th, 13th, and 14th LBs, while a pilot (which is also called a Reference Signal (RS)) is transmitted at each of the 4th and 11th LBs. The pilot includes pre-promised sequences, and is thus used in channel estimation for coherent demodulation in a receiver side.
In the LTE system, when only control information is transmitted in an uplink, the control information is transmitted through a pre-defined control information frequency band. In the present specification, this type of transmission scheme is called a “type A” transmission scheme. In the “type A” transmission scheme, the number of LBs for transmission of control information and the number of LBs for transmission of the RS, which are included in the control information frequency band, may change according to the case. Referring to FIG. 1, the control information frequency band corresponds to the RUs 110 and 112 located at both ends of the system transmission band 114.
In general, a frequency band for transmitting control information is configured RU-by-RU, and a plurality of RUs are used for transmission of control information according to the number of User Equipments (UEs) to be multiplexed. Further, frequency hopping may be employed in order to increase frequency diversity during one sub-frame, wherein the frequency hopping may be performed for each slot.
Referring to FIG. 1, control information #1 is transmitted through a pre-allocated frequency band 110 in the first slot 104, and is frequency-hopped and then transmitted through another pre-allocated frequency band 112 in the second slot 106. Not shown, control information #2 is transmitted through the frequency band 112 in the first slot 104, and is frequency-hopped and then transmitted through the frequency band 110 in the second slot 106.
A Code Division Multiplexing (CDM) scheme may be used in order to multiplex uplink control information including ACK/NACK information, CQI information, MIMO information, etc. between different users. The CDM scheme is more robust against an interference signal than the Frequency Division Multiplex (FDM) scheme.
The Zadoff-Chu (ZC) sequence is being discussed as a sequence to be used for a CDM scheme of control information. Since the Zadoff-Chu sequence has a constant signal level (constant envelope) in the time and frequency domain, the Zadoff-Chu sequence has a good Peak to Average Power Ratio (PAPR) and shows a good channel estimation performance in the frequency domain.
The Zadoff-Chu sequence has zero circular autocorrelation for a non-zero shift. Therefore, UEs using the same Zadoff-Chu sequence for transmission of control information can be given different time domain cyclic shift values of the Zadoff-Chu sequence in order to discriminate between the UEs. The cyclic shift values are set to be different according to users and to be larger than the maximum transmission delay value of a wireless transmission path, so as to maintain orthogonality between the users. Therefore, the number of users capable of having multiple access is determined by the length of the Zadoff-Chu sequence and the cyclic shift values.
Hereinafter, mapping and transmission of a control information signal and a Zadoff-Chu sequence in the “type A” transmission scheme will be described with reference to FIG. 1. Provided that a Zadoff-Chu sequence having a length of N allocated to UE i is defined by g(n+Δi)mod N (n=0, . . . , N−1, Δi indicates a time domain cyclic shift value for UE i, and i indicates a UE index for identifying a UE) and a control information signal to be transmitted by UE i is denoted by mi,k (k=0, . . . , NLB, wherein NLB refers to the number of LBs within a sub-frame), a signal Ci,k,n (the nth sample of kth LB of UE i) mapped to each LB is defined by Equation (1) below:Ci,k,n=g(n+Δi)mod N·mi,k  (1)
In Equation (1), k=0, . . . , NLB, n=0, . . . , N−1, and Δi indicates a time domain cyclic shift value of a Zadoff-Chu sequence for UE i.
In the structure shown in FIG. 1, NLB indicating the number of LBs within one sub-frame is 12, and the length N of a Zadoff-Chu sequence is also 12, which is equal to the number of sub-carriers included in one RU. In FIG. 1, the UE index i is omitted. In a view of one UE, a time domain cyclic-shifted Zadoff-Chu sequence is applied to each LB, and a control information signal to be transmitted is configured by multiplying the time domain cyclic-shifted Zadoff-Chu sequence by one modulation symbol for each LB. Therefore, a maximum number of NLB control information modulation symbols for each sub-frame can be transmitted. That is, in the sub-frame shown in FIG. 1, a maximum of 12 control information modulation symbols can be transmitted.
When both control information and data are transmitted, the data and the control information are time-division-multiplexed, mapped to time-frequency resources allocated for transmission of the data, and then transmitted. In the present specification, this type of transmission scheme is called a “type B” transmission scheme. In general, a Node B schedules the time-frequency resources RU by RU. FIG. 2 illustrates a structure of control information transmitted according to the “type B” transmission scheme in a 3GPP LTE system. For a system transmission bandwidth 208, one sub-frame 202 has a length of 1 ms, and includes two slots 204 and 206, each of which has a length of 0.5 ms. Each of the slots includes seven LBs 218.
Referring to FIG. 2, within one sub-frame 202, control information and data are time division multiplexed and transmitted at the 1st, 2nd, 3rd, 5th, 6th, 7th, 8th, 9th, 10th, 12th, 13th, and 14th LBs, while an RS is transmitted at each of the 4th and 11th LBs. Further, within the transmission bandwidth 208, frequency bands 214 and 216 have been allocated for transmission of “type A” control information. Therefore, it is possible to use the “type B” scheme for transmission of control information in frequency bands other than the frequency bands 214 and 216. UE #1 time division multiplexes and transmits control information and data in the frequency band 210, and UE #2 time division multiplexes and transmits control information and data in the frequency band in the frequency band 212.
As described above, in transmitting uplink control information, UEs employ the “type B” scheme or the “type A” scheme according to whether there is uplink data to be transmitted together with the uplink control information. However, when there is a large quantity of control information to be transmitted, resources for the time domain, the frequency domain, and the code domain may be insufficient.
The quantity of control information varies according to the type of the control information. CQI information for feeding back a downlink channel state is described below as an example. The CQI information includes a wideband CQI indicating a channel state of the entire system transmission band and a sub-band CQI indicating a channel state of a particular frequency band. A Node B performs a scheduling operation for determining resources to be allocated to a UE based on CQI information fed back from the UE. Frequency selective scheduling requires a sub-band CQI. The system transmission band includes a plurality of sub-bands, and each of the sub-bands has a size corresponding to a multiple of the RU, which is the smallest unit of scheduling by the Node B.
Given a 10 MHz transmission band, an LTE system may employ a total of 50 RUs, each of which includes 12 sub-carriers. If each sub-band includes two RUs, the LTE system includes a total of 25 sub-bands and thus a UE feeds back 25 sub-band CQIs. In general, in considering the signaling overhead, it is preferable to feed back CQI information for a part of sub-bands having the best channel condition from among all sub-bands. For example, if it is assumed that a sub-band CQI is fed back for three sub-bands having the best channel condition from among the 25 sub-bands and each sub-band CQI is indicated by 5 bits, the number of all signaling bits necessary for feeding back all sub-band CQI information is calculated as follows. That is, a total of 27 bits, which include 12 bits (=ceil{log2(25C3)}) for indicating what sub-band the CQI information relates to, and 15 bits (=5*3) for indicating the channel state of each sub-band, are necessary in order to feed back all sub-band CQI information, wherein ceil { } refers to a ceiling function.
In view of scheduling, it is preferable to transmit the sub-band CQI information for each minimum transmission time unit with as short a transmission delay as possible. When a convolutional coding having a coding rate of 1/3 and applying 8 tail bits is performed, an encoded stream including 105 bits (=(27+8)*3 bits) is generated. Then, if the encoded stream undergoes a Quadrature Phase Shift Keying (QPSK) modulation, 52.5 modulation symbols (=105/2) are generated.
In considering that a maximum of 12 modulation symbols can be transmitted according to the “type A” scheme within one sub-frame in the case of example shown in FIG. 1, it is necessary to define a transmission scheme for a case where the quantity of information to be transmitted (52.5 modulation symbols) is larger than the quantity of transmissible information (12 modulation symbols).